An automated manual transmission to which a shift position sensor of the present invention is practically applied has a hydraulic shift actuator that includes a cylinder and a piston that moves in the cylinder in accordance with a speed change operation of the transmission. The shift position sensor comprises a magnet mounted in the piston to move therewith, and a magnaflux detector mounted to a fixed member of the transmission near the hydraulic shift actuator to detect a magnaflux produced by the magnet. The cylinder is so constructed as to cover the magnet even when the piston is subjected to a stroke in the cylinder.